This invention relates to bonding diamond or cubic boron nitride (CBN) to metal substrates, and also to improved products such as semiconductor device heat sinks and industrial grinding and cutting tools in which diamond or CBN is bonded to a metal supporting structure.
Because diamond is the non-metal with the highest thermal conductivity over a usable range of temperatures, diamond has been used as a substrate to heat sink microwave and other semiconductor power devices. CBN also has a high thermal conductivity approximating that of diamond and could also be used in such applications. Diamond and CBN particles are also employed as grinding wheel abrading elements and are subject to high temperatures under working conditions. Similarly, bonded polycrystalline compacts of diamond or CBN are used as cutting tool blanks and inserts which are exposed to high temperatures in use. Such compacts are disclosed e.g., in U.S. Pat. Nos. 3,136,615, 3,233,988, 3,743,489, 3,745,623 and 3,767,371. In order to more successfully use diamond and CBN as heat sinks or to extract the heat from diamond or CBN abrading or cutting elements, it is desirable to have a strong, high thermal conductivity bond between the diamond or CBN and a metal substrate or supporting structure.
Although silver is known to have the highest thermal conductivity of the metals, a difficulty with pure silver used as a brazing material is that it does not adhere well to diamond. The more commonly used "silver solders" or "silver brazing alloys" actually are alloy compositions with considerable less than 100 percent silver which do not have the desired combination of properties. The prior art relating to bonding diamond to metal bases also refers to depositing very thin layers of platinum or cobalt on diamond as preparation for further steps, and broadly refers to silver coatings, but the deficiency of all of these is that either the thermal conductivity or the tensile strength of the bond to diamond is low.